1. Field of the Invention
The present invention relates to optical coatings on glass or other substrates and more particularly to coatings which control phase changes between the s- and p-plane polarization components of a transmitted beam of light.
2. Description of the Prior Art
The use of multi-layered dielectric coatings on glass surfaces to reduce reflectivity is well known in the prior art. U.S. Pat. No. 3,858,965, entitled Five Layer Anti-Reflection Coating, issued to Haruki Sumita on Jan. 7, 1975, teaches a multi-layered anti-reflection coating for use with a glass substrate which includes five layers of coating material each of which has an optical thickness of a preselected design wavelength and an index of refraction in a particular range. The optical thicknesses of the layers can be varied to compensate for any variations from the theoretical design indices of refraction. The multi-layered anti-reflection coating substantially reduces reflection from the surface of the glass substrate over a broad wavelength band. Although classical designs for anti-reflecting light of only one wavelength have been well established in the prior art, the novelty of this anti-reflection coating is that the layer thicknesses of its multi-layered configuration can be calculated using a computer in conjunction with a least squares algorithm in order to provide a broad band anti-reflection coating.
U.S. Pat. No. 4,142,958, entitled Method for Fabricating Multi-Layer Optical Films, issued to David T. Wei and Anthony W. Louderback on Mar. 6, 1979, teaches a method of fabricating multiple layer interference optical films by ion beam sputtering. The films are used for mirrors in a ring laser apparatus. The layers alternate between a material of high index of refraction, such as either tantalum pentoxide (Ta.sub.2 O.sub.5) or titanium dioxide (TiO.sub.2), and a material of low index of refraction, such as silicon dioxide (SiO.sub.2), i.e., quartz. U.S. Pat. No. 2,519,722, entitled Metallic Mirror and Method of Making Same, issued to Arthur F. Turner on Aug. 22, 1950, teaches superimposed light-transmitting layers of non-metallic materials having low and high indices of refraction which are deposited alternately on a metallic reflecting surfaces so that each layer has a thickness of approximately the thickness of a quarter wave length of light in the portion of the spectrum from about 500 m.mu. to 600 m.mu.. Quarter wave stacks and their design are explained in detail in the Military Standardization Handbook entitled "Optical Designs," MIL-HDBK-141, Oct. 5, 1962. Briefly, each layer or thin film coating in a quarter wave stack has a thickness of about one quarter of a wavelength of the light which the quarter wave stack is designed to reflect. Multilayer dielectric coatings can also be used to enhance the reflection of either glass or metallic surfaces. Since metallic surfaces are already good reflectors, only a few layers, such as either two or four layers, are needed to bring reflectivity to ninety-nine percent or higher. For glass surfaces, twenty or more layers are often used to produce highly reflective surfaces.
More recently, multilayer dielectric coating configurations have been invented which not only provide enhanced reflectivity for metals, but which also provide a means to produce a 90.degree. phase shift between the s- and p-plane polarization components. The invention also utilized a computer to calculate the layer thicknesses to achieve the desired reflectivity and phase shift. No prior art had existed that predicted such a phase shift could be achieved. Since that invention, however, some attempts have been made to develop a theory for the reflective phase retarders. It should be noted that this theory pertains only to mirrors. In an article, entitled "Multilayer coating producing 90.degree. phase change," published in Applied Optics, volume 18, number 11, on June 1, 1979, page 1875, William H. Southwell has discussed enhanced reflection dieletric coating for a metallic reflector. In another article, entitled "Multilayer coating design achieving a broadband 90.degree. phase shift," published in Applied Optics, volume 19, number 16, on Aug. 15, 1980, pages 2688-2692, William H. Southwell has discussed the broadband 90.degree. phase shift. In an article, entitled "Graphical method to design multilayer phase retarders," published in Applied Optics, volume 20, number 6, Mar. 15, 1981, pages 1024-1029, and an article, entitled "Phase retardance of periodic multilayer mirrors," published in Applied Optics, volume 21, number 4, pages 733-738, Joseph H. Apfel has discussed the theory of the formation of phase retarders.
U.S. Pat. No. 4,312,570, entitled High Reflectivity Coated Mirror Producing 90 Degree Phase Shift, issued to William H. Southwell on Jan. 26, 1982, teaches a high reflectivity mirror which produces a substantially 90 phase shift between s- and p-plane polarization components of the reflected light by applying a plurality of superimposed transparent layers on the reflective surface of a substrate. Adjacent layers are made out of materials of substantially different indices of refraction. The thickness of substantially all of the layers is less than a quarter wavelength at the center frequency of the incident light and the thickness of the layers differ from each in a predetermined manner to control and produce exactly 90.degree. phase shift between the s- and p-plane polarization components while providing maximum reflectivity over a wide frequency band.
A phenomenon is known that when a light beam is reflected by the surface of a certain substance a phase difference is created between a s-plane polarization reflected component and an p-plane polarization reflected component. For example, when a light beam is reflected by the outer surface of a dielectric material a phase difference .pi. is created between the p-plane polarization reflected component and the s-plane polarization reflected component if the angle of incidence is smaller than the Brewster's angle. In the metals generally used for mirrors, namely aluminum, silver and chromium when a light beam is reflected by the surface thereof a phase difference of approximately .pi. is created between the s-plane polarization reflected component and the p-plane polarization reflected component. In this case, the angle of incidence at which the light beam is incident on the metal surface is about 45.degree., which is usually a standard value when the metal is used as a mirror.
U.S. Pat. No. 4,322,130, entitled Phase Shifting Mirror, issued to Susumu Ito and Mikichi Ban on Mar. 30, 1982, teaches a phase shifting mirror which includes a thin film layer of metal disposed on a substrate and a thin film layer of dieletric material disposed on the thin film layer of metal wherein the utilization of a phase difference between a s-plane polarization reflected component and an p-plane polarization reflected component reflecting from the reflection boundary surface between the thin film layer of metal and the thin film layer of dieletric material and variations in refractive index and film thickness of the dieletric material a desired phase difference is obtained between the s-plane polarization reflected component and the p-plane polarization reflected component.
U.S. Pat. No. 4,084,883, entitled Reflective Polarization Retarder and Laser Apparatus Utilizing Same, issued to Jay Morgan Eastman and Stanley J. Refermat on Apr. 18, 1979, teaches a reflective thin film polarization retardation device which permits obtaining phase retardation of light different amounts of which may be readily obtained. Retardation results from interference effects within thin film arrays within the device. A polarizer rotator includes a thin film reflective transmissive polarizer, a thin film reflector and a phase adjusting layer, which may be one or more thin film layers. The phase adjusting layer is sandwiched between the thin film polarizer and the thin film reflector. The thin films constituting the polarizer, phase adjusting layer and reflector may be successively deposited on of one of the planar faces of a substrate formed from laser glass with the films constituting the polarizer being deposited first. The s-plane polarization component of the incident light, which may be linearly polarized laser light, is reflected by the polarizer and the p-plane polarization component of the incident light is transmitted through the polarizer and the phase adjusting layer to the reflector. In the device which U.S. Pat. No. 4,084,883 teaches the approach is to use two classical concepts, a thin film polarizer and a spacer layer to provide a phase retardation in order to derive a phase shift. There is no necessity to use a computer in order to calculate the layer thicknesses in that there is a phase adjusting layer. U.S. Pat. No. 2,519,722 teaches the use of dielectric coatings on polished metals or other reflective surfaces to improve the reflectivity in which superimposed light-transmitting layers of nonmetallic materials are used having alternately low and high indices of refraction. The thickness of these thin films is held to a quarter wavelength of the incident light beam. Use of the thin film polarizers give the advantage of economy of manufacture, large apertures and broadband wavelength performance characteristics. A disadvantage, however, is that the beam has to bend since the phase retarders are on mirrors.
Previously birefringent crystals were required to form transmissive phase retarders. Quarter wave and ha1f wave plates are examples of devices which performed phase retardations between two polarization components in a beam of light. The advantage of these crystals is that the beam does not have to bend since the beam travels through them. The disadvantage of these crystals is that they are not only expensive, but they are not useful for higher power laser beams. R. T. Denton, in a Chapter C6, entitled "Modulation Techniques", of Laser Handbook, published by North-Holland Publishing Company in 1972, described several commonly used crystals which are formed out of electo-optic materials, such as cadmium telluride (CdTe) and gallium arsenide (GaAs), and which have static birefringence such that phase retardation occurs even in the absence of an applied voltage. The waves travel through the crystals at different velocities.
U.S. Pat. No. 3,591,188, entitled Internally Modulated Laser, issued to Thomas A. Nussmeier on July 13, 1971, teaches an internal modulator which modulates a laser beam and which incorporates in a single element a polarizer and a retardation modulator. The retardation modulator is formed from a gallium arsenide crystal.
It is desirable to have a phase retarding device which has all of the advantages of the reflective phase retarders, but which would not deviate the light beam namely a transmissive phase retarder. It would also be desirable to provide tunability meaning that by a simple adjustment, such as a rotation of the device in a light beam so as to change the angle of incidence, a prescribed phase retardation could be achieved.
The present invention is an optical multilayer coating which reduces the unwanted reflection from the surface and which also produces a phase shift between the s- and p-plane polarization components of the transmitted light. The glass or other transparent substrate must be placed at some non-normal incidence angle to the beam of light in order to define the s- and p-components of an electric field. An additional feature of the coating designs described by this invention is that the adjustment of the angle of incidence provides the basis for a variable phase retardation device. By the use of this invention namely, the use of a computer to determine coating layer thicknesses so as to provide the anti-reflective properties as well as the desired phase shift properties, designs have been developed which allow the user of the device to select the phase retardation between 0.degree. and 100.degree. by adjusting the angle of incidence.
For information regarding mica and quartz retardation plates the inventor directs attention to Section 10, entitled "Polarization," of the Handbook of Optics, written by Jean M. Bennett and Harold E. Bennett, edited by Walter G. Driscoll and William Vaughan, published by McGraw-Hill Book Company in 1978.